Two evangelists have become the unlikely standard bearers for free speech in Australia as they fight a council ban on their preaching.

The brothers' fire and brimstone sermons had become a regular piece of street theatre on Friday evenings in Adelaide's CBD.

Brothers Caleb and Samuel Corneloup used the popular shopping precinct of Rundle Mall as a pulpit, preaching their version of the gospel and attracting protests from those opposed to their views.

But what started as a passionate public debate over morality has found its way to the High Court, where it could become a judgment on freedom of speech.

It was retailers who first called for action against the street preachers, telling the Adelaide City Council that the weekly fracas was turning away shoppers.

"We have had complaints, many complaints from traders and from passersby who felt that they had been spoken to in an unreasonable manner," Adelaide City Council's Peter Smith said.

The city initially fined the pair and then modified council bylaws to silence them by denying them a permit to preach.

The brothers challenged the decision in court, explaining they believe public speaking is the only avenue open to them.

"Tolerance for certain things like homosexuality, the cultivating of homosexuality in the schools - it's not just tolerated, it's cultivated," Caleb Corneloup said outside court in August 2010.

"So the only thing that we've got, the only chance that we've got is to preach in the streets, because everywhere else we're banned."

Their aggressive evangelism continued most notably when they interrupted an anti-homophobia rally.

But soon after this clash the brothers had a victory in the Supreme Court when it ruled some of the council bylaw changes were invalid because they prevented free political communication, which is the closest thing Australia has to a constitutional right to freedom of speech.

Freedom of speech

Ralph Bonig, president of the Law Society of South Australia, says freedom of speech is an implied right.

"We're looking at, on the one hand, the right to freedom of speech and the ability to express yourself, and the other hand the ability of lawmakers to control in some way where you should be expressing yourself, and maybe what you can and can't say," he said.

"When we look at what we can and cannot do as a country, we need from time to time to imply those rights. So freedom of speech is an implied right that has been recognised by the High Court.

"And what it means is that we can go about and express ourselves from a political or a religious viewpoint without fear of prosecution or without fear of having it reined in as long as we don't overstep those rules such as defamation and harassment."

On Tuesday morning, the South Australian Government appealed to the High Court with the support of the attorneys-general of Queensland, NSW, Victoria, Western Australia and the Commonwealth

They believe what is at stake is the ability of a city council to pass laws to regulate its own streets.

But others, including Anna Brown from the Human Rights Law Centre, see it differently.

"I think it's important to remember that whatever one thinks of the street preachers and their views, it's not those views that are on trial," she said.

"The court's been asked to consider not the content of the preachers' views but the question of whether the bylaws, as drafted, limit freedom of speech in a way that breaches the Australian Constitution."

The Human Rights Law Centre is making a submission in the case and likens it to the Occupy Melbourne protests that grew violent last year.

"They both raise similar and significant issues of human rights protected under the Australian Constitution," Ms Brown said.

"The Occupy case and this case both concern protestor rights, but importantly they will have broader implications for the right of all Australians to gather in groups and express their views in public spaces."

Caleb and Samuel Corneloup were in court yesterday, but they do not give interviews because they say they have been misquoted too often.